Effect of salinity on arsenic uptake, biotransformation, and time-dependent speciation pattern by Sargassum species.

The arsenic (As) content of seaweed has been extensively studied due to its toxicological concerns. As a primary producer, seaweed plays a vital role in the biochemical cycling of As in marine environments. Several studies have focused on the growth and behavior of seaweed under a salinity gradient; however, information related to the impact of salinity on As uptake, biotransformation mechanism, and time-dependent speciation patterns of these plants is limited. This study aimed to investigate the temporal effects of salinity on these factors in seaweed. Three seaweed species, Sargassum fusiforme, Sargassum thunbergii, and Sargassum horneri, were maintained in a 1% Provasoli-enriched seawater medium for 14 d under 5‰, 15‰, 25‰, and 34‰ salinities. The results revealed that the high salinity media promoted a rapid uptake of As by all three species. Arsenic accumulation inside the cell approached 100% within seven days of culture for S. thunbergii, irrespective of the salinity content of the media. In addition, As(V) biotransformation and release by S. fusiforme and S. thunbergii were time-dependent, while S. horneri released dimethylarsinic acid (DMAA) from day 3 of the culture. All seaweed species showed methylation of As(V) to DMAA during the culture period. Furthermore, S. thunbergii released DMAA when As(V) was completely depleted from the culture media, whereas the release by S. fusiforme and S. horneri was relatively earlier than that of S. thunbergii. S. horneri showed minimal tolerance to low salinity, as the cells revealed significant damage. Based on the results of this study, a conceptual model was developed that demonstrated the effects of salinity on As uptake and the biotransformation mechanism of seaweed.

A new evaluation system of iron bioavailability in seaweed.

In marine ecosystems, the avid binding of iron (Fe) to organic ligands influences Fe bioavailability in seaweed. This study aimed to elucidate Fe's biological availability to seaweed and develop a simple and rapid bioassay method as a new evaluation system. Undaria pinnatifida was used as a model seaweed species and the actual seaweed samples were collected using the 0.5 m × 0.5 m quadrat from the Mashike Bay area of Hokkaido, Japan. Chlorophyll fluorescence measurements were utilized as an index to evaluate the biological -effectiveness of Fe and compared with the results of culture tests based on growth. The effect of Fe content on media, pre-culture, concentrations and types of chelating and reducing agents in clearing solutions, cleaning time, Fe removal effect, and resistance to seaweed were systematically optimized to obtain the maximum efficacy of the washing solution. A bioassay was developed to evaluate the Fe environment by combining chlorophyll fluorescence measurements. The findings suggest that the tolerance of seaweeds to the wash solution is strongly influenced by the concentrations of the chelating and reducing agents than their types. Washing with 0.02 M Ti-Citrate/EDTA solution for 80 s was the most effective in terms of maximum Fe removal with minimum cell damage. The application of pre-culture and chemical pre-treatment methods under Fe deficiency to the culture strain confirmed the maximum reproducibility in the culture test. Finally, the developed method was applied to actual seaweed samples and was found to be applicable to many seaweed species. However, the method was less robust for some seaweed species and depended on the seaweed growth stage.